The present invention relates to a brake system for automotive vehicles of the type comprising a vacuum brake force booster operatively located between a brake pedal and a master brake cylinder which includes at least two working chambers separated from one another by a membrane plate, with one of the working chambers, through a master magnetic valve, being in communication with a vacuum source, and the other of the working chambers, through a control valve actuable by the brake pedal, being ventable to generate a brake force in proportion to the brake pedal force. Such a system further includes brake circuits in communication with the master brake cylinder for applying pressure to wheel brake cylinders. Sensors associated with the wheels detect the rotational behavior of the wheels to identify a locking condition, with the output signals thereof being applied to a central electronic regulator supplying, in response to the input signals, at least one switch-over signal to the master magnetic valve such that the working chamber connected to the vacuum source can be placed in communication with the atmosphere rather than with the vacuum source.
A brake system for automotive vehicles of the afore-described type is disclosed in the published German application DE-OS No. 33 17 629 or its corresponding U.S. Pat. No. 4,702,531 wherein a vacuum brake force booster is provided to transfer the brake pedal force generated by the brake pedal, through a piston-and-push rod, to a tandem master brake cylinder. The vacuum brake force booster comprises a vacuum working chamber separated, by a membrane plate, from a ventable working chamber, with the membrane plate being fixed to the piston-and-push rod. The tandem master brake cylinder, through two brake circuits, is in communication with corresponding wheel brake cylinders of the brakes.
During normal deceleration, first, the brake pedal force, through the piston-and-push rod, is transferred to the tandem master brake cylinder. At the same time, the rear working chamber of the vacuum brake force booster is vented by means of a control valve, while the front working chamber continues to be connected to the vacuum source such that different pressures prevail on the two sides of the membrane plate. The pressure difference on the membrane plate, accordingly, causes an auxiliary force to be transferred along with the brake pedal force as a brake force to the master brake cylinder.
When a brake skid control is required during deceleration because of the locking condition of one of the decelerated wheels as detected by a device for monitoring the rotating pattern of the wheels, the brake force acting upon the master brake cylinder will have to be reduced. For that purpose, the vacuum working chamber is vented while the rear pressure working chamber is connected to the vacuum source and, accordingly, is evacuated. The pressure difference acting upon the membrane plate, first, is reduced until the auxiliary force is eliminated. In the event that a further reduction in the brake pressure is required, a pressure difference directed opposite to the pressure prevailing in the normal operational sense is developed on the membrane plate causing, through the membrane plate, a compensation firce to be applied opposite the brake pedal force such that the master brake cylinder can be further relieved. When brake skid control is no longer required, the normal connections of the vacuum brake force booster are re-established thereby enabling the braking operation to be performed in usual manner.
Although in this prior known brake system for automotive vehicles, brake skid control is performed very rapidly, it is imperative in the interest of an increasing safety of automotive vehicles to enable the brake pressure as generated by the master brake cylinder to be decreased even more rapidly than such prior art systems.